The invention relates to coating and extruding dies. More specifically, the invention relates to the configuration of the coating or extruding orifices.
Coating and extruding dies with continuous slots are expensive to manufacture and set. Machining and setup costs of continuous slot fluid bearing dies are large. Maintaining a uniform feed slot is beneficial in that fluid exiting the slot maintains a continuous cross-sectional profile. In order to maintain uniformity of the feed slot in the cross-web direction, dies have to be large and require elaborate mounting setup to provide adequate structural support around the slot. “Cross-web” direction is generally defined as the width dimension of a substrate (typically a web of paper or polymeric material) translating with respect to the die. The “cross-web” direction is perpendicular to the direction of travel of the web with respect to the die. “Cross-web” direction may be used to explain a direction of the die, the coating on a web, an extrudate or the web itself.
Dies with multiple orifices provided a less expensive alternative to continuous slot dies. Multiple orifice dies had a number of openings that allow the fluid (e.g. liquid) to exit the die distribution chamber. In order to provide a continuous cross-sectional profile of the fluid, fluid translating through the die was merged using external lands or troughs after it passed through the orifices. For example, in fluid bearing dies, downstream of where the fluid exited the die, a portion of the die was used to merge individual fluid streams into a continuous fluid coating on a web (often referred to as a “smoothing land”). Typically, a downstream portion of the smoothing land ended with a sharp edge, used to prevent ribbing and gaps in the coating. The length of the smoothing land is normally measured in the downstream direction, from the orifices to the sharp edge. Other types of dies combined the streams using a “trough” which collected and merged the fluid inside the die before the fluid was coated. Examples of multiple orifices are illustrated and described in U.S. Pat. Nos. 3,149,949; 4,774,109; 5,045,358; and 4,371,571, all of which are incorporated by reference in their entirety, herein.
Because neighboring fluid streams from these previous multiple orifice type dies must be merged before coating (or extruding) on a web or other substrate in order to form a continuous cross-sectional profile of the fluid, previous dies of this type have somewhat narrow range of coating (or extrusion) parameters (e.g., line speed, die settings, desired thickness of coated (or extruded) film, die position, etc.) in order to provide a coated (or extruded) layer which is continuous, smooth, and bubble free. This is due to the techniques required to merge the separate fluid streams created by the adjacent orifices. Especially troublesome is the merging point of the two streams. Air often becomes entrained between the fluid and the substrate at that point, which can cause imperfections in the end product.